1. Field of the Invention
The present invention relates to a small size power amplifier, more specifically to a small size power amplifier having a function of switching output power.
2. Description of the Related Art
Wireless communication performed by mobile phones or the like faces a problem of lack in a frequency band because of increase of the number of users and capacity increase in data communication. In order to improve frequency utilization efficiency for solving such a problem, a higher speed process and capacity increase have been in progress by using digital modulation techniques in recent years. In order to meet a strict requirement for the linearity of a signal in the digital modulation, operation with excellent linearity is required in power amplifiers used in wireless communication.
However, a power amplifier generally exhibits a high power-added-efficiency when operating nonlinearly in a high power state close to saturation output power, but power consumption relatively increases in operation with excellent linearity. Particularly, when the amplifier capable of maintaining linearity at high output power is used at a low output power, efficiency is extremely reduced. In a third-generation mobile phone system, a ratio of an operation time in a high output power of the power amplifier to an operation time in a low output power is small, though depending on modulation systems and purposes. As shown in FIG. 1, more operation time is spent in the low output power operation with a relatively large power consumption.
On the other hand, a mobile terminal is required to achieve a longer battery driving time and a smaller size. Though power consumption increases because of a higher terminal functionality, there is also a strong requirement for reduction of power consumption of a power amplifier which carries out wireless communication. For this reason, highly efficient operation needs to be achieved at low output power because of the above background.
Output power and power-added-efficiency of a power amplifier are determined based on load impedance, i.e., impedance when viewing an external circuit from the output end of the power amplifier. Generally, if the load impedance is decreased, the saturation output power can be set a high output power state. However, when a predetermined output power is desired, a high power-added-efficiency can be attained by increasing the load impedance and decreasing a saturation output power of the power amplifier to a desired output power (a smaller backoff amount state). In other words, as shown in FIG. 2, the high power-added-efficiency can be attained when the output power is immediately before the saturation output power of the power amplifier, while the efficiency is decreased if a desired output power is lower than the saturation output power (a larger backoff amount state). Usually, since a single amplifier is used from a low output power to a high output power, the saturation output power is designed to be compatible with the high output power. For this reason, an operation with low power-added-efficiency is unavoidable at low output power.
Moreover, it is necessary to enlarge the size of power amplifier for the high output power, and a bias (or idle) current is also increased at the same time, so that the power-added-efficiency is decreased particularly at low output power.
From these backgrounds, various techniques are proposed in which amplifiers are switched between a small amplifier having increased load impedance for a highly efficient operation at low output power and a large amplifier having decreased load impedance for highly efficient operation at high output power. Although there are proposed a method of using an externally attached switch and a method of using a λ/4 line, there is a problem in increase of a mount area in case of the externally attached parts. Therefore, it is desired to provide output power switching function suitable for miniaturization.
It should be noted that a technique to switch RF signal paths by use of a switch is known as a technique to switch a high efficient power amplifier at low output power and a high efficient power amplifier at high output power. However, in this technique, the number of parts, mounting area and costs are increased.
The Doherty amplifier shown in FIG. 3 is known as a circuit configuration to allow an efficiency improvement at low output power without using a switch. The Doherty amplifier is configured by arranging in parallel, a main amplifier which is biased to a class AB to operate from low output power and a peak amplifier which is biased to a class C to operate only at large output power. When an input power is small, the peak amplifier does not operate and only the main amplifier operates, and a load impedance is set to allow a highly efficient operation at low output power. When the input power is increased, the peak amplifier starts operating and performs a highly efficient operation at high output power. In order to realize these operations, impedance conversion is carried out by inserting a λ/4 line in the output of one of the amplifiers so as to achieve an optimum load impedance in the peak amplifier for high output power and in the main amplifier for low output power. Moreover, in order to prevent generation of a phase difference between outputs signals, a λ/4 line is inserted into the input of the other amplifier. Due to this configuration, the Doherty amplifier is effective in the power amplifier which needs to operate highly efficiently at low output power, but generally regarded as a circuit unsuitable for miniaturization due to the necessity of λ/4 lines.
Accordingly, another configuration is proposed in which the λ/4 line on an input side is omitted for miniaturization while utilizing operation principle of the Doherty amplifier. To be more specifically, as shown in FIG. 4, a circuit configuration example includes a port 100, a driver amplifier 110, a node 130, an impedance matching network 140, a peak amplifier 150, a λ/4 impedance conversion network 170, a node 180, an impedance matching network 190, and a port 200. Here, a first path is assumed be a signal path from the port 100 to the port 200 via the driver amplifier 110, the node 130, the impedance matching network 140, the peak amplifier 150, the node 180, and the impedance matching network 190. A second path is assumed to be a signal path from the port 100 to the port 200 via the driver amplifier 110, the node 130, the λ/4 impedance conversion network 170, and the impedance matching network 190. In the circuit shown in FIG. 4, the driver amplifier 110 is biased to a class AB and the peak amplifier 150 is biased to a class C. In a low RF input signal, load impedance to determine an output power of the driver amplifier is determined based on the λ/4 impedance conversion network 170 and designed to be highly efficient at low output power. Output power of the driver amplifier is increased and the peak amplifier which is biased to a class C starts operating. Load impedance to determine an output power of the peak amplifier is designed to be highly efficient at high output power. Due to the above operation principle, a highly efficient operation is realized at both low output power and high output power. This configuration has an advantage that while two of λ/4 lines are required in general Doherty amplifier, one of λ/4 lines can be omitted by arranging the driver amplifier and the peak amplifier in series.
A circuit similar to the above circuit is disclosed in Japanese Laid Open Patent application (JP-P2005-244862A: Related Art 1). FIG. 5 shows a configuration example of the circuit. As shown in FIG. 5, this circuit configuration example includes the port 100, the driver amplifier 110, an impedance matching network 120, the node 130, the impedance matching network 140, the peak amplifier 150, the impedance matching network 160, the λ/4 impedance conversion network 170, the node 180, the impedance matching network 190, the port 200, and a voltage control circuit 250. Here, a first path is assumed to be a signal path from the port 100 to the port 200 via the driver amplifier 110, the impedance matching network 120, the node 130, the impedance matching network 140, the peak amplifier 150, the impedance matching network 160, the node 180, and the impedance matching network 190. A second path is also assumed to be a signal path from the port 100 to the port 200 via the driver amplifier 110, the impedance matching network 120, the node 130, the λ/4 impedance conversion network 170, and the impedance matching network 190. Moreover, the peak amplifier 150 is connected to the voltage control circuit 250. The voltage control circuit 250 biases the peak amplifier 150 in accordance with an operation mode.
In conventional techniques, the peak amplifier is biased to a class C in order to dynamically switch an operation of the peak amplifier in accordance with input power. However, in the Related Art 1, the peak amplifier is not biased at low output power (is not operated) and is biased at the high output power by explicitly switching the operation mode. That is, the peak amplifier 150 is biased to an operation state at high output power, and an impedance when being viewed from the node 130 at this time is designed to be lower impedance in the impedance matching network 140 than that of the impedance conversion network 170. Accordingly, the RF signal is amplified by the peak amplifier and outputted to the port 200. The voltage control circuit 250 decreases a bias to the peak amplifier 150 at low output power to prevent the operation of the amplifier 150. Also, impedance when viewing the impedance matching network 140 from the node 130 is high so that the RF signal is outputted to the port 200 via the impedance conversion network 170.
The above circuit in the Related Art 1 is suitable for miniaturization since an externally attached member such as a switch is not used. Also, the circuit achieves suppression of power consumption since the peak amplifier is not biased at low output power. In this technique, like the Doherty amplifier, a technique is used which amplifiers are switched in accordance with output power by separating signal paths without using any switch.
As described above, in the Related Art 1, a circuit is proposed in which the number of λ/4 lines is decreased to one by arranging a main amplifier and a peak amplifier in series while applying Doherty amplifiers. The main amplifier is employed as a driver amplifier in this circuit, and the peak amplifier is bypassed at low output power, so that a function as a one-stage amplifier is exhibited. On the contrary, a function of two-stage amplifiers is exhibited because amplification operation is made in the peak amplifier in response to an output of the driver amplifier at high output power. Therefore, a significant discontinuity is observed in the entire power amplifier gain in the switching between the modes. Thus, even in Doherty amplifier or the switching by use of an RF switch, since gain differences exist between the peak amplifier having a load impedance for high power-added-efficiency at high output power and the main amplifier having a load impedance for high power-added-efficiency at low output power, a gain adjustment is required in general. In addition, when the power amplifier is actually used, it is required to control output power to a desired value. Therefore, an unnecessary control is generally required in the amplifier which has a significant discontinuity in the gain and a method of using the amplifier is limited.